Improve readability of Kotlin code (#1236)

* style(kotlin): Improve kotlin codes readability.

* remove redundant quotes.

* style(kotlin): improve codes readability.

codes/kotlin/chapter_hashing/built_in_hash.kt

@@ -11,15 +11,15 @@ import utils.ListNode
 /* Driver Code */
 fun main() {
     val num = 3
-    val hashNum = Integer.hashCode(num)
+    val hashNum = num.hashCode()
     println("整数 $num 的哈希值为 $hashNum")
 
     val bol = true
-    val hashBol = Boolean.hashCode()
+    val hashBol = bol.hashCode()
     println("布尔量 $bol 的哈希值为 $hashBol")
 
     val dec = 3.14159
-    val hashDec = java.lang.Double.hashCode(dec)
+    val hashDec = dec.hashCode()
     println("小数 $dec 的哈希值为 $hashDec")
 
     val str = "Hello 算法"